Opposition leaders tonight said their candidate, Viktor Yushchenko, had pulled out of negotiations on Ukraine's disputed presidential election, and demanded that MPs return to parliament tonight for an emergency no-confidence vote in the government.

"We hereby announce that we have pulled out of the negotiating process. We are renewing the blockade of the government building and are demanding an emergency session of parliament at 8 or 9pm [1800 or 1900 GMT]," Oleksander Zinchenko was quoted by the Interfax-Ukraine news agency as telling journalists.

"We ask all deputies to be ready to participate and urge all our supporters to mobilise," he said.

Another opposition leader, Mykola Tomenko, told the crowds of Mr Yushchenko's supporters massed in the main square of the capital, Kiev, that if the parliament did not vote to dismiss the government of Moscow-back prime minister Viktor Yanukovich, the opposition would consider that political means had been exhausted and civil resistance would begin.

The warning followed an earlier attempt by opposition supporters to storm parliament. Mr Yushchenko's supporters, who had been watching proceedings on television screens outside the building, reacted angrily to moves to annul Saturday's non-binding decision to declare the November 21 presidential poll invalid.

A small number of protesters got as far as the lobby of the building before police pushed them back.

The parliamentary speaker, Volodymyr Lytvyn, adjourned the session until tomorrow. Seeking to soothe passions, he promised parliament would not cancel its previous decision and called on the opposition not to try to seize the building.

Mr Lytvyn called parliament "the last pillar of democracy" and said "tomorrow we will make a decision that will satisfy all of Ukraine", Reuters reported.

Mr Yushchenko also came out to address the demonstrators in an effort to calm tensions. Reuters reported that opposition officials had told protesters to restore their blockade of official buildings except for parliament "to give it a chance to make a decision" and told them to return to Kiev's main square.

The parliament had been due to debate a no-confidence motion in the government, which would have increased pressure on Mr Yanukovich.

The Moscow-backed Mr Yanukovich was last week declared the official winner of the presidential elections while his rival, the western-leaning Mr Yushchenko, has claimed the vote was rigged. Thousands of opposition supporters were out on the streets of Kiev for an eighth consecutive day today.

It seemed increasingly likely that a new presidential election is the most likely resolution to the bitter political dispute and will avert splitting the country into a pro-Russian east and pro-Europe west.

Leonid Kuchma, the country's outgoing president, yesterday backed the idea of a new poll and today Russia agreed to respect the results if another vote took place. The opposition has demanded assurances that any new vote would be held quickly.

The speaker, Mr Lytvyn criticised "many state, regional and local officials in the east and south of the country for inciting separatism ... and unconstitutional and illegal aspirations for independence and autonomy".

He said calls for new autonomous regions in Ukraine were "illegal and punishable under the criminal code".

The Russian parliamentary speaker, Boris Gryzlov, was among those predicting a split. Today Mr Gryzlov, who took part in last week's meeting between Mr Yushchenko and Mr Yanukovich, told reporters: "The situation there is heading towards a split or towards bloodshed ... I see no other way the situation could develop."

Meanwhile, Ukraine's supreme court continued to consider an opposition appeal to annul the election results, which said Mr Yanukovich had won by a margin of 871,402 votes. Under Ukrainian legislation, the court cannot rule on the overall results, but can declare results invalid in individual precincts.

The appeal focuses on the results from eight eastern and southern regions, accounting for more than 15m votes - almost half of the total cast in the runoff.

Opposition officials are asking the court to declare Mr Yushchenko the winner, based on his winning a narrow plurality of the votes in the first round on October 31.

The west has refused to recognise the election results, while Russia, which still yields considerable influence over Ukraine, congratulated Mr Yanukovich on his apparent victory.

Mr Yanukovich yesterday said he would support a new vote if allegations of fraud were proven, but added he had yet to see such proof.

Meanwhile the country's central bank was forced to tighten banking controls to prevent a potentially devastating rush on banks for hard currency. The central bank limited dollar sales to companies and individuals and set limits on withdrawals from cash machines.